Method and apparatus for antenna selection in a diversity antenna system for communicating with implantable medical device

CROSS-REFERENCE TO RELATED APPLICATION

p-0002This application is related to co-pending, commonly assigned, U.S. patent application Ser. No. 11/068,497, entitled “DIVERSITY ANTENNA SYSTEM FOR COMMUNICATION WITH AN IMPLANTABLE MEDICAL DEVICE,” filed on Feb. 28, 2005 and U.S. patent application Ser. No. 11/068,476, entitled “METHOD AND APPARATUS FOR OPERATING A DIVERSITY ANTENNA SYSTEM COMMUNICATING WITH IMPLANTABLE MEDICAL DEVICE” filed on Feb. 28, 2005, which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

p-0003This document relates generally to telemetry for implantable medical systems and particularly to an external telemetry system having diversity antennas for communicating with an implantable medical device.

BACKGROUND

p-0004Medical devices are implanted in human bodies for monitoring physiological conditions, diagnosing diseases, treating diseases, or restoring functions of organs or tissues. Examples of such implantable medical devices include cardiac rhythm management (CRM) devices, neural stimulators, neuromuscular stimulators, drug delivery devices, and biological therapy devices. When an implantable medical device is intended for long-term use in a patient, its size and power consumption are limited by implantability and longevity requirements. Consequently, many implantable medical devices depend on external systems to perform certain functions. Communication between an implantable method device and an external system is performed via telemetry. Examples of specific telemetry functions include programming the implantable medical device to perform certain monitoring or therapeutic tasks, extracting an operational status of the implantable medical device, transmitting real-time physiological data acquired by the implantable medical device, and extracting physiological data acquired by and stored in the implantable medical device.

p-0005One type of telemetry between the implantable medical device and the external system is based on inductive coupling between two closely-placed coils using the mutual inductance between these coils. One of the coils is part of the implantable medical device, and the other coil is part of the external system. This type of telemetry is referred to as inductive telemetry or near-field telemetry because the coils must be closely situated for obtaining magnetically coupled communication.

p-0006Far-field radio-frequency (RF) telemetry provides another means for communications between the implantable medical device and the external system. The far-field RF telemetry is performed using an RF transceiver in the implantable medical device and an RF transceiver in the external system. The far-field RF telemetry frees the patient from any body surface attachment that limits mobility and is more suitable for use when the patient is at home, without the attendance by the physician or other professional caregiver.

p-0007The far-field RF telemetry between the implantable medical device and the external system often operates in an environment where RF electromagnetic waves are reflected from various kinds of surfaces. Destructive interference between the incident and reflective waves results in nulls, where the incident wave and reflected wave cancel out. The far-filed RF telemetry link is substantially interrupted when an antenna encounters a null. While such a null is moving and usually transient, the interruption to the telemetry link may last long enough to cause a data transmission error.

p-0008Therefore, there is a need for ensuring the quality of far-field RF telemetry between an external system and an implanted device when nulls are present.

SUMMARY

p-0009A far-field RF telemetry system for communicating with an implantable medical device includes a diversity antenna system. An antenna control circuit selects one or more antennas of the diversity antenna system for reducing potential data transmission errors associated with nulls.

p-0010In one embodiment, an external system communicating with an implantable medical device includes a diversity antenna system, a transceiver, an antenna interface circuit, and an antenna control circuit. The diversity antenna system includes a plurality of antennas for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The transceiver transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The antenna interface circuit includes a switch circuit that connects an antenna of the diversity antenna system to the transceiver according to an antenna selection signal. The antenna control circuit produces the antenna selection signal and includes a fading detector, an antenna selector, and an antenna switching timing circuit. The fading detector detects a transmission failure deemed to be associated with a null. The antenna selector adjusts the antenna selection signal for connecting a different antenna of the diversity antenna system to the transceiver in response to a detection of the transmission failure. The antenna switching timing circuit holds the antenna selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received.

p-0011In one embodiment, a method is provided for operating a telemetry system communicating with an implantable medical device. Using a diversity antenna system including a plurality of antennas, an outgoing signal modulated by outgoing data frames is transmitted to the implantable medical device, and an incoming signal modulated by incoming data frames is received from the implantable medical device. An active antenna is selected from the diversity antenna system according to an antenna selection signal. A transmission failure deemed to be associated with a null is detected. In response to a detection of the transmission failure, the antenna selection signal is adjusted for selecting a different active antenna of the diversity antenna system. The antenna selection signal is held while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. When no data frame is being transmitted or received, the different active antenna is selected from the diversity antenna system according to the adjusted antenna selection signal.

p-0012In one embodiment, a telemetry system for communicating with an implantable medical device includes a diversity antenna system, a transceiver, an antenna interface circuit, and an antenna control circuit. The diversity antenna system includes a plurality of antennas for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The transceiver transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The antenna interface circuit includes a switch circuit that connects an antenna of the diversity antenna system to the transceiver according to an antenna selection signal. The antenna control circuit includes a selection sequence generator that produces the antenna selection signal for selecting an antenna of the diversity antenna system to be connected to the transceiver in response to an antenna switching timing signal.

p-0013In one embodiment, a method is provided for operating a telemetry system communicating with an implantable medical device. Using a diversity antenna system including a plurality of antennas, an outgoing signal modulated by outgoing data frames is transmitted to the implantable medical device, and an incoming signal modulated by incoming data frames is received from the implantable medical device. An active antenna is selected from the diversity antenna system according to an antenna selection signal. The antenna selection signal is produced for selecting a new active antenna of the diversity antenna system on a predetermined periodic basis.

p-0014In one embodiment, an external system communicating with an implantable medical device includes a diversity antenna system, an external system controller, a transceiver, and an antenna control circuit. The diversity antenna system includes a plurality of antennas for transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The transceiver transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The transceiver includes a plurality of receiving modules and a switch circuit. The receiving modules each have an input coupled to an antenna of the diversity antenna system and an output. The switch circuit connects the output of one of the receiving modules to the external system controller according to a receiving path selection signal. The antenna control circuit produces the receiving path selection signal and includes a signal quality assessment circuit and a receiving path selector. The signal quality assessment circuit produces an indication of quality for the incoming signal processed by each of the receiving modules. The receiving path selector adjusts the receiving path selection signal based on the indications of quality for the incoming signal produced for the plurality of receiving modules.

p-0015In one embodiment, a method is provided for operating a telemetry system communicating with an implantable medical device. Using a diversity antenna system including a plurality of antennas, an outgoing signal modulated by outgoing data frames is transmitted to the implantable medical device, and an incoming signal modulated by incoming data frames is received from the implantable medical device. The incoming signal is processed through a plurality of processing paths each coupled to an antenna of the diversity antenna system. An indication of quality is produced for the incoming signal processed by each of the processing paths. One of the processing paths is selected based on the detected indications of quality associated with the processing paths.

p-0016This Summary is an overview of some of the teachings of the present application and not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details about the present subject matter are found in the detailed description and appended claims. Other aspects of the invention will be apparent to persons skilled in the art upon reading and understanding the following detailed description and viewing the drawings that form a part thereof. The scope of the present invention is defined by the appended claims and their legal equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

p-0017In the drawings, which are not necessarily drawn to scale, like numerals describe similar components throughout the several views. The drawings illustrate generally, by way of example, various embodiments discussed in the present document.

p-0018<figref idrefs="DRAWINGS">FIG. 1</figref> is an illustration of an embodiment of a CRM system including an implantable medical device and an external system and portions of an environment in which the CRM system is used.

p-0019<figref idrefs="DRAWINGS">FIG. 2</figref> is a block diagram illustrating a specific embodiment of the external system.

p-0020<figref idrefs="DRAWINGS">FIG. 3</figref> is a block diagram illustrating an embodiment of a telemetry system of the external system.

p-0021<figref idrefs="DRAWINGS">FIG. 4</figref> is a block diagram illustrating a specific embodiment of the telemetry system of <figref idrefs="DRAWINGS">FIG. 3</figref>.

p-0022<figref idrefs="DRAWINGS">FIG. 5</figref> is a block diagram illustrating another specific embodiment of the telemetry system of <figref idrefs="DRAWINGS">FIG. 3</figref>.

p-0023<figref idrefs="DRAWINGS">FIG. 6</figref> is a block diagram illustrating another specific embodiment of the telemetry system of <figref idrefs="DRAWINGS">FIG. 3</figref>.

p-0024<figref idrefs="DRAWINGS">FIG. 7</figref> is a block diagram illustrating another specific embodiment of the telemetry system of <figref idrefs="DRAWINGS">FIG. 3</figref>.

p-0025<figref idrefs="DRAWINGS">FIG. 8</figref> is a block diagram illustrating another specific embodiment of the telemetry system of <figref idrefs="DRAWINGS">FIG. 3</figref>.

p-0026<figref idrefs="DRAWINGS">FIG. 9</figref> is a block diagram illustrating another embodiment of the telemetry system of the external system.

p-0027<figref idrefs="DRAWINGS">FIG. 10</figref> is a block diagram illustrating another embodiment of the telemetry system of the external system.

p-0028<figref idrefs="DRAWINGS">FIG. 11</figref> is a flow chart illustrating a method for operating a telemetry system communicating with an implantable medical device.

p-0029<figref idrefs="DRAWINGS">FIG. 12</figref> is a flow chart illustrating another method for operating a telemetry system communicating with the implantable medical device.

p-0030<figref idrefs="DRAWINGS">FIG. 13</figref> is a flow chart illustrating another method for operating a telemetry system communicating with the implantable medical device.

DETAILED DESCRIPTION

p-0031In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their legal equivalents.

p-0032It should be noted that references to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment.

p-0033This document discusses an RF telemetry system for bi-directional communication between an implantable medical device and an external system. The external system includes an external telemetry system that uses a diversity antenna system and an antenna control circuit to selecting one or more active antenna of the diversity antenna system for reducing or minimizing data transmission errors associated with nulls. An active antenna is an antenna that is currently used to transmit and/or receive signals. In one embodiment, a different active antenna is selected when a transmission failure such as a data transmission error or a sudden signal strength drop is detected. Such a transmission failure is deemed to be associated with a null resulting from destructive interference between the incident and reflected electromagnetic waves. A null is a point where the destructive interference causes a substantial loss of the RF telemetry including interruption of data communication. In another embodiment, a new antenna is selected to be the active antenna on a regular basis, such as on a periodic basis. This reduces the probability for a currently active antenna to encounter a null. In another embodiment, the telemetry system includes multiple processing paths each associated with an antenna of the diversity antenna system. A different processing path is selected when the transmission failure is detected.

p-0034<figref idrefs="DRAWINGS">FIG. 1</figref> is an illustration of an embodiment of portions of a CRM system <b>100</b> and portions of an environment in which system <b>100</b> is used. System <b>100</b> includes an implantable medical device <b>110</b> and an external system <b>120</b>. In the illustrated embodiment, after being implanted into a patient's body <b>101</b>, implantable medical device <b>110</b> is coupled to the patient's heart <b>102</b> through a lead system <b>105</b>. Examples of implantable medical device <b>110</b> include pacemakers, cardioverter/defibrillators, cardiac resynchronization therapy (CRT) devices, cardiac remodeling control therapy (RCT) devices, neural stimulators, drug delivery systems, biological therapy devices, and patient monitoring devices. External system <b>120</b> allows a physician or other caregiver to interact with implantable medical device <b>110</b> through an RF telemetry link <b>115</b>, which provides for bi-directional data communication between implantable medical device <b>110</b> and external system <b>120</b>.

p-0035RF telemetry link <b>115</b> provides for data transmission from implantable medical device <b>110</b> to external system <b>120</b>. This includes, for example, transmitting real-time physiological data acquired by implantable medical device <b>110</b>, extracting physiological data acquired by and stored in implantable medical device <b>110</b>, extracting therapy history data stored in implantable medical device <b>110</b>, and extracting data indicating an operational status of implantable medical device <b>110</b> (e.g., battery status and lead impedance). RF telemetry link <b>115</b> also provides for data transmission from external system <b>120</b> to implantable medical device <b>110</b>. This includes, for example, programming implantable medical device <b>110</b> to acquire physiological data, programming implantable medical device <b>110</b> to perform at least one self-diagnostic test (such as for a device operational status), and programming implantable medical device <b>110</b> to deliver at least one therapy.

p-0036RF telemetry link <b>115</b> is a far-field telemetry link. A far-field, also referred to as the Fraunhofer zone, refers to the zone in which a component of an electromagnetic field produced by the transmitting electromagnetic radiation source decays substantially proportionally to 1/r, where r is the distance between an observation point and the radiation source. Accordingly, far-field refers to the zone outside the boundary of r=λ/2π, where λ is the wavelength of the transmitted electromagnetic energy. In one embodiment, a communication range of RF telemetry link <b>115</b> (a distance over which data is capable of being wirelessly communicated) is at least ten feet but can be as long as allowed by the communication technology utilized. Unlike an inductive telemetry link using a coil placed near implantable medical device <b>110</b>, attached to the patient, and electrically connected to external system <b>120</b> with a cable, using RF telemetry link <b>115</b> frees the patient from any physical restraints caused by the coil and the cable and allows external system <b>120</b> to be placed entirely away from the sterile filed during an operation such as the implantation of implantable medical device <b>110</b>.

p-0037Telemetry link <b>115</b> is supported by an implant telemetry system of implantable medical device <b>110</b> and an external telemetry system <b>122</b> of external system <b>120</b>. External telemetry system <b>122</b> includes a diversity antenna system <b>126</b>, an antenna interface circuit <b>128</b>, a transceiver <b>130</b>, and an antenna control circuit <b>132</b>. Diversity antenna system <b>126</b> includes a plurality of antennas to transmit an outgoing signal to implantable medical device <b>110</b> and to receive an incoming signal from implantable medical device <b>110</b>. Antenna interface circuit <b>128</b> includes tuning circuitry for diversity antenna system <b>126</b> and routes the outgoing and incoming signals between diversity antenna system <b>126</b> and transceiver <b>130</b>. Transceiver <b>130</b> transmits outgoing data frames by modulating the outgoing signal and receives incoming data frames by demodulating the incoming signal. The outgoing data frames and the incoming data frames are each a frame being a logic unit of data including a header, a payload, and a trailer. The header includes a “comma,” which includes a unique set of bits for signaling a receipt of a frame. A lack of comma, or failure to receive the comma, indicates a failure to receive a frame. The payload includes the data block being transmitted. The trailer includes a cyclic redundancy check (CRC) character having a value generated by a transmitter. A receiver receives that CRC character and also recalculates the CRC character based on the received data block and compares the result to the received CRC character in the trailer. The data is deemed to be correctly transmitted if the recalculated CRC character matches the received CRC character. A CRC error refers to a mismatch between the recalculated CRC character and the received CRC character. Depending on the specific communication formats, the header and the trailer each include additional information for flagging, control of data recovery, and/or synchronization of the receiving device. Antenna control circuit <b>132</b> controls the operation of antenna interface circuit for an approximately optimal performance, or at least an acceptable performance, of diversity antenna system <b>126</b>. In one embodiment, antenna control circuit <b>132</b> selects an active antenna of diversity antenna system <b>126</b> or a processing path associated with an antenna of diversity antenna system <b>126</b> based on the quality of the outgoing signal and/or the incoming signal. Such quality is measured by, for example, strength of signal and/or integrity of the data frames. In a further embodiment, antenna control circuit <b>132</b> selects a different active antenna of diversity antenna system <b>126</b> or a different processing path associated with an antenna of diversity antenna system <b>126</b> in response to the detection of a transmission failure, such as a sudden drop in signal strength or a data transmission error. Such a transmission failure is deemed to be associated with a null. In another further embodiment, antenna control circuit selects a new active antenna of diversity antenna system <b>126</b> or a new processing path associated with an antenna of diversity antenna system <b>126</b> on a periodic basis to reduce the probability for diversity antenna system <b>126</b> to encounter a null. External telemetry system <b>122</b> is connected to an external system controller <b>124</b>, which allows external system <b>120</b> to receive information acquired by implantable medical device <b>110</b> and to control the operation of implantable medical device <b>110</b>. External system controller <b>124</b> receives the incoming data frames from transceiver <b>130</b> and sends the outgoing data frames to transceiver <b>130</b>. A user interface <b>125</b> allows the physician or other caregiver to view the received information and to enter commands and parameters to control the operation of CRM system <b>100</b>.

p-0038In one embodiment, external system <b>120</b> includes a programmer. In another embodiment, as illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>, external system <b>120</b> includes a patient management system.

p-0039<figref idrefs="DRAWINGS">FIG. 2</figref> is a block diagram illustrating an embodiment of external system <b>220</b>, which is a specific embodiment of external system <b>120</b>. As illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref>, external system <b>220</b> is a patient management system including an external device <b>234</b>, a telecommunication network <b>236</b>, and one or more remote devices <b>238</b>. External device <b>234</b> is placed within the vicinity of implantable medical device <b>110</b> and includes external telemetry system <b>122</b> to communicate with implantable medical device <b>110</b> via telemetry link <b>115</b>. Remote device(s) <b>238</b> are in one or more remote locations and communicates with external device <b>234</b> through network <b>236</b>, thus allowing a physician or other caregiver to monitor and treat a patient from a distant location and/or allowing access to various treatment resources from the one or more remote locations.

p-0040<figref idrefs="DRAWINGS">FIG. 3</figref> is a block diagram illustrating an embodiment of an external telemetry system <b>322</b>, which is a specific embodiment of external telemetry system <b>122</b>. External telemetry system <b>322</b> includes a diversity antenna system <b>326</b>, an antenna interface circuit <b>328</b>, a transceiver <b>330</b>, and an antenna control circuit <b>332</b>.

p-0041Diversity antenna system <b>326</b> is a specific embodiment of diversity antenna system <b>126</b> and includes two or more antennas <b>340</b>A-N. Each of antennas <b>340</b>A-N allows for transmitting an outgoing signal to implantable medical device <b>110</b> and/or receiving an incoming signal from implantable medical device <b>110</b>. In one embodiment, diversity antenna system <b>126</b> includes two antennas. In one specific embodiment, the two antennas are mounted onto a chassis of an external device such as the programmer or external device <b>234</b>. In other embodiments, diversity antenna system <b>126</b> includes three or more antennas. One example of diversity antenna system <b>126</b> is discussed in U.S. patent application Ser. No. 11/068,497, entitled “DIVERSITY ANTENNA SYSTEM FOR COMMUNICATION WITH AN IMPLANTABLE MEDICAL DEVICE,” filed on Feb. 28, 2005, which is incorporated by reference herein in its entirety.

p-0042Antenna interface circuit <b>328</b> is a specific embodiment of antenna interface circuit <b>128</b> and includes tuning circuits <b>342</b>A-N and a switch circuit <b>344</b>. Tuning circuits <b>342</b>A-N each provide tuning for a corresponding antenna of antennas <b>342</b>A-N. Switch circuit <b>344</b> provides for a controllable connection between an antenna of diversity antenna system <b>326</b> and transceiver <b>330</b> according to an antenna selection signal. In one embodiment, switch circuit <b>344</b> substantially completes the change of the connection from between an antenna of diversity antenna system <b>326</b> and transceiver <b>330</b> to between another antenna of diversity antenna system <b>326</b> and transceiver <b>330</b> in about 50 microseconds to 1 millisecond.

p-0043Transceiver <b>330</b> is a specific embodiment of transceiver <b>130</b> and includes a modulator <b>346</b> and a demodulator <b>348</b>. Modulator <b>346</b> produces the outgoing signal by modulating an RF carrier with the outgoing data frames. In one embodiment, the frequency of the RF carrier for the outgoing signal is in a range of approximately 902 MHz to 928 MHz, with approximately 914 MHz being one specific example. The data transmission rate for the outgoing signal is in a range of approximately 60 kilobits per second to 500 kilobits per second, with approximately 204.8 kilobits per second being one specific example. Demodulator <b>348</b> recovers the incoming data frames by demodulating the received incoming signal. The implant telemetry circuit of implantable medical device <b>110</b> produces the incoming signal by modulating another RF carrier with the incoming data frames. In one embodiment, the frequency of the RF carrier for the incoming signal is in a range of approximately 902 MHz to 928 MHz, with approximately 914 MHz being one specific example. The data transmission rate for the incoming signal is in a range of approximately 60 kilobits per second to 500 kilobits per second, with approximately 102.4 kilobits per second being one specific example. In one embodiment, amplitude-shift-keying (ASK) is the modulation scheme used for both the outgoing signal and the incoming signal. Modulator <b>346</b> is an ASK modulator, and demodulator <b>348</b> is an ASK demodulator.

p-0044Antenna control circuit <b>332</b> is a specific embodiment of antenna control circuit <b>132</b> and includes a fading detector <b>350</b>, an antenna selector <b>352</b>, and an antenna switching timing circuit <b>354</b>. Fading detector <b>350</b> detects a transmission failure deemed to be associated with a null. Exemplary specific embodiments of fading detector <b>350</b> are discussed below with reference to <figref idrefs="DRAWINGS">FIGS. 4-8</figref>. Antenna selector <b>352</b> adjusts the antenna selection signal for connecting a different antenna of diversity antenna system <b>326</b> to transceiver <b>330</b> in response to a detection of the transmission failure. Antenna switching timing circuit <b>354</b> holds the antenna selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. Thus, after antenna selector <b>352</b> adjusts the antenna selection signal, the change of connection between diversity antenna system <b>326</b> and transceiver <b>330</b> occurs when no data frame is being transmitted or received. This prevents a potential data transmission error from being resulted from the change of connection, i.e., switching from one antenna to another. In one embodiment, antenna switching timing circuit <b>354</b> delays the adjustment of the antenna selection signal by antenna selector <b>352</b> until an ongoing transmission or reception of a data frame is completed. In another embodiment, antenna switching timing circuit <b>354</b> keeps the adjusted antenna selection signal from being applied to switch circuit <b>344</b> until an ongoing transmission or reception of a data frame is completed. In one embodiment, antenna switching timing circuit <b>354</b> holds the antenna selection signal while a data frame is being transmitted or received only if the switching time of switch circuit <b>344</b> for switching from one antenna to another is not substantially higher than the time required for transmitting one data bit. In another embodiment, external telemetry system <b>322</b> includes an error protection circuit to prevent data transmission errors caused by an operation of switching circuit <b>344</b> in response to a change in the antenna selection signal. The error protection circuit corrects a detected error in received incoming data frames by executing an error correction algorithm.

p-0045<figref idrefs="DRAWINGS">FIG. 4</figref> is a block diagram illustrating an embodiment of an external telemetry system <b>422</b>, which is a specific embodiment of external telemetry system <b>322</b>. External telemetry system <b>422</b> includes diversity antenna system <b>326</b>, antenna interface circuit <b>328</b>, transceiver <b>330</b>, and an antenna control circuit <b>432</b>.

p-0046Antenna control circuit <b>432</b> is a specific embodiment of antenna control circuit <b>332</b> and includes an incoming frame failure detector <b>450</b> and an antenna selector <b>452</b>. Incoming frame failure detector <b>450</b> is a specific embodiment of fading detector <b>350</b> and detects an incoming frame failure as the transmission failure from the incoming signal. The incoming frame failure includes a data transmission error in at least one of the incoming data frames. In one embodiment, a single data transmission error in one incoming data frame constitutes the transmission failure deemed to be associated with a null. Incoming frame failure detector <b>450</b> includes a CRC failure detector <b>456</b> and/or a lack-of-comma detector <b>458</b>. CRC failure detector <b>456</b> detects a CRC failure from the incoming signal and indicates the incoming frame failure if the CRC failure is detected. Lack-of-comma detector <b>458</b> detects a comma indicative of a receipt of an incoming data frame during a predetermined time window and indicates the incoming frame failure if the comma is not detected during the predetermined time window. In other embodiments, incoming frame failure detector <b>450</b> includes one or more error detector detecting data transmission errors of types other than the CRC failure and the lack of comma. Generally, incoming frame failure detector <b>450</b> detects any predetermined type data transmission error in at least one of the incoming data frames and indicates the incoming frame failure when the predetermined type data transmission error is detected.

p-0047Antenna selector <b>452</b> is a specific embodiment of antenna selector <b>352</b> and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system <b>326</b> to transceiver <b>330</b> when the incoming frame failure is indicated by incoming frame failure detector <b>450</b>. In one embodiment, antenna selector <b>452</b> adjusts the antenna selection signal in response to the detection of either a CRC failure or a lack of comma. In response to the adjusted antenna selection signal, switch circuit <b>344</b> connects the different antenna to transceiver <b>330</b> for receiving the incoming signal and transmitting the outgoing signal until the detection of another incoming frame failure is indicated.

p-0048<figref idrefs="DRAWINGS">FIG. 5</figref> is a block diagram an embodiment of an external telemetry system <b>522</b>, which is another specific embodiment of external telemetry system <b>322</b>. External telemetry system <b>522</b> includes diversity antenna system <b>326</b>, antenna interface circuit <b>328</b>, transceiver <b>330</b>, and an antenna control circuit <b>532</b>.

p-0049Antenna control circuit <b>532</b> is a specific embodiment of antenna control circuit <b>332</b> and includes a response failure detector <b>550</b> and an antenna selector <b>552</b>. Response failure detector <b>550</b> is a specific embodiment of fading detector <b>350</b> and detects a response failure as the transmission failure from the incoming signal. The response failure includes a data transmission error in at least one response frame of the incoming data frames. The response frame is an incoming data frame produced and sent by implantable medical device <b>110</b> in response to an outgoing data frame transmitted to implantable medical device <b>110</b>. In one embodiment, a single data transmission error in one response frame constitutes the transmission failure deemed to be associated with a null. Response failure detector <b>550</b> includes a CRC failure detector <b>556</b> and/or a lack-of-response detector <b>558</b>. CRC failure detector <b>556</b> detects a CRC failure from the incoming signal and indicates the response failure if the CRC failure is detected. Lack-of-response detector <b>558</b> detects a response frame indicative of a receipt of the outgoing data frame transmitted to implantable medical device <b>110</b> and indicates the response failure if no response frame is detected during a predetermined time window starting from the transmission of the outgoing data frame.

p-0050Antenna selector <b>552</b> is a specific embodiment of antenna selector <b>352</b> and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system <b>326</b> to transceiver <b>330</b> when the response failure is indicated. In one embodiment, antenna selector <b>552</b> adjusts the antenna selection signal in response to the detection of either a CRC failure or a lack of response. In response to the adjusted antenna selection signal, switch circuit <b>344</b> connects the different antenna to transceiver <b>330</b> for receiving the incoming signal and transmitting the outgoing signal until the detection of another response failure is indicated.

p-0051<figref idrefs="DRAWINGS">FIG. 6</figref> is a block diagram illustrating an embodiment of an external telemetry system <b>622</b>, which is another specific embodiment of external telemetry system <b>322</b>. External telemetry system <b>622</b> includes diversity antenna system <b>326</b>, antenna interface circuit <b>328</b>, transceiver <b>330</b>, and an antenna control circuit <b>632</b>.

p-0052Antenna control circuit <b>632</b> is a specific embodiment of antenna control circuit <b>332</b> and includes a signal strength failure detector <b>650</b> and an antenna selector <b>652</b>. Signal strength failure detector <b>650</b> is a specific embodiment of fading detector <b>350</b> and detects a signal strength failure as the transmission failure from the incoming signal. The signal strength failure as detected by signal strength failure detector <b>650</b> is deemed to be caused by a null. Signal strength failure detector <b>650</b> includes a signal strength detector <b>660</b>, a signal strength averaging circuit <b>661</b>, a threshold generator <b>662</b>, and a comparator <b>664</b>. Signal strength detector <b>660</b> measures a strength parameter being a measure of the strength of the incoming signal. In one embodiment, the strength parameter is a power measured in dBm (decibel ratio (log <b>10</b>) of watts (W) to one milliwatt (1 mW)), and signal strength detector <b>660</b> includes a signal power detector. In another embodiment, the strength parameter is an amplitude measured in volts, and signal strength detector <b>660</b> includes a signal amplitude detector. Signal strength averaging circuit <b>661</b> calculates an average value for the measured strength parameter. In one embodiment, signal strength averaging circuit <b>661</b> calculates the average value for the strength parameter over a predetermined period of time. In another embodiment, signal strength averaging circuit <b>661</b> calculates an average value for the strength parameter over a predetermined number of frames. In a specific embodiment, the predetermined number is in a range of 4 to 50 frames, with approximately 6 frames being a specific example. Threshold generator <b>662</b> dynamically produces a threshold strength based on the average value for the measured strength parameter. In one embodiment, the threshold strength is produced by subtracting a predetermined margin from the average value for the measured strength parameter. In one specific embodiment, the predetermined margin is in a range of approximately 10 dBm to 30 dBm, with approximately 20 dBm being a specific example. Comparator <b>664</b> includes an input to receive the average value for the measured strength parameter, another input to receive the dynamically produced threshold strength, and an output indicative of the signal strength failure when the average value for the measured strength parameter falls below the dynamically produced threshold strength.

p-0053Antenna selector <b>652</b> is a specific embodiment of antenna selector <b>352</b> and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system <b>326</b> to transceiver <b>330</b> when comparator <b>664</b> indicates the signal strength failure. In one embodiment, antenna selector <b>652</b> adjusts the antenna selection signal immediately in response to an indication of the signal strength failure. In another embodiment, antenna selector <b>652</b> adjusts the antenna selection signal if the signal strength failure is indicated for a predetermined time interval in a range of approximately 20 milliseconds to 200 milliseconds, with approximately 50 milliseconds being a specific example. In response to the adjusted antenna selection signal, switch circuit <b>344</b> connects the different antenna to transceiver <b>330</b> for receiving the incoming signal and transmitting the outgoing signal until the detection of another signal strength failure is indicated.

p-0054<figref idrefs="DRAWINGS">FIG. 7</figref> is a block diagram illustrating an embodiment of an external telemetry system <b>722</b>, which is another specific embodiment of external telemetry system <b>322</b>. External telemetry system <b>722</b> includes diversity antenna system <b>326</b>, antenna interface circuit <b>328</b>, transceiver <b>330</b>, and an antenna control circuit <b>732</b>.

p-0055Antenna control circuit <b>732</b> is a specific embodiment of antenna control circuit <b>332</b> and includes a signal strength failure detector <b>750</b> and an antenna selector <b>752</b>. Signal strength failure detector <b>750</b> is another specific embodiment of fading detector <b>350</b> and detects a signal strength failure associated with the outgoing signal as the transmission failure. Signal strength failure detector <b>750</b> includes a signal strength receiver <b>765</b> and/or an acknowledgement signal receiver <b>766</b>. Signal strength receiver <b>765</b> receives a reporting frame being an incoming data frame produced and sent by implantable medical device <b>110</b>, which detects the signal strength failure in receiving the outgoing frames. Signal strength receiver <b>765</b> indicates the signal strength failure when the reporting frame includes data indicating a signal strength failure associated with the outgoing signal. Acknowledgement signal receiver <b>766</b> receives an acknowledgement frame of the incoming data frames and indicates the signal strength failure if the acknowledgement frame is not received within a predetermined time interval after a transmission of an outgoing frame to implantable medical device <b>110</b>. The acknowledgement frame is indicative of a successful receipt of the outgoing frame by implantable medical device <b>110</b>.

p-0056Antenna selector <b>752</b> is a specific embodiment of antenna selector <b>352</b> and adjusts the antenna selection signal for connecting a different antenna of diversity antenna system <b>326</b> to transceiver <b>330</b> when the signal strength failure is indicated. In one embodiment, antenna selector <b>752</b> adjusts the antenna selection signal in response to either a receipt of a data frame indicative of a signal strength failure in outgoing signal as received by implantable medical device <b>110</b> or a lack of the acknowledgement frame. In response to the adjusted antenna selection signal, switch circuit <b>344</b> connects the different antenna to transceiver <b>330</b> for receiving the incoming signal and transmitting the outgoing signal until the detection of another signal strength failure is indicated.

p-0057<figref idrefs="DRAWINGS">FIG. 8</figref> is a block diagram an embodiment of an external telemetry system <b>822</b>, which is another specific embodiment of external telemetry system <b>322</b>. External telemetry system <b>822</b> includes diversity antenna system <b>326</b>, antenna interface circuit <b>328</b>, transceiver <b>330</b>, and an antenna control circuit <b>832</b>.

p-0058Antenna control circuit <b>832</b> is a specific embodiment of antenna control circuit <b>332</b> and includes a signal strength failure detector <b>850</b> and an antenna selector <b>852</b>. Signal strength failure detector <b>850</b> is another specific embodiment of fading detector <b>350</b> and detects signal strength failures associated with the incoming signal and the outgoing signal. Signal strength failure detector <b>850</b> includes an incoming signal strength failure detector <b>868</b> and an outgoing signal strength failure detector <b>870</b>. Incoming signal strength failure detector <b>868</b> detects a signal strength failure associated with the incoming signal as the transmission failure and indicates an incoming signal strength failure when the signal strength failure is detected. In one embodiment, incoming signal strength failure detector <b>868</b> is substantially identical or similar to signal strength failure detector <b>650</b>. Outgoing signal strength failure detector <b>870</b> detects signal strength failures associated with the outgoing signal as the transmission failure and indicates an outgoing signal strength failure when the signal strength failure is detected. In one embodiment, outgoing signal strength failure detector <b>870</b> is substantially identical or similar to signal strength failure detector <b>750</b>.

p-0059Antenna selector <b>852</b> includes a receiving antenna selector <b>872</b> and a transmitting antenna selector <b>874</b>. Receiving antenna selector <b>872</b> adjusts the antenna selection signal for selecting a different antenna of diversity antenna system <b>326</b> for receiving the incoming signal when a detection of the incoming signal strength failure is indicated. Transmitting antenna selector <b>874</b> adjusts the antenna selection signal for selecting a different antenna of diversity antenna system <b>326</b> for transmitting the outgoing signal when a detection of the outgoing signal strength failure is indicated. In one embodiment, the antenna selection signal allows selection of two different antennas: one for receiving the incoming signal and the other for transmitting the outgoing signal. In another embodiment, the antenna selection signal allows selection of either one antenna, or two different antennas, for receiving the incoming signal and transmitting the outgoing signal.

p-0060In various other specific embodiments, fading detector <b>350</b> selectively includes one or more elements of incoming frame failure detector <b>450</b>, response failure detector <b>550</b>, signal strength failure detector <b>650</b>, signal strength failure detector <b>750</b>, and signal strength failure detector <b>850</b>. In one exemplary embodiment, fading detector <b>350</b> includes incoming frame failure detector <b>450</b> (or portions thereof) and signal strength failure detector <b>650</b> (or portions thereof). Antenna selector <b>452</b> adjusts the antenna selection signal in response to the detection of either an incoming frame failure or a signal strength failure. In another embodiment, fading detector <b>350</b> includes incoming frame failure detector <b>450</b>, response failure detector <b>550</b>, and signal strength failure detector <b>850</b>. Antenna selector <b>452</b> adjusts the antenna selection signal in response to the detection of any of an incoming frame failure, a response failure, an incoming signal strength failure, and an outgoing signal strength failure. Other embodiments involving such embodiments will become apparent to those skilled in the art upon reading and understanding this document.

p-0061<figref idrefs="DRAWINGS">FIG. 9</figref> is a block diagram illustrating an embodiment of an external telemetry system <b>922</b>, which is another specific embodiment of external telemetry system <b>122</b>. External telemetry system <b>922</b> includes diversity antenna system <b>326</b>, antenna interface circuit <b>328</b>, transceiver <b>330</b>, and an antenna control circuit <b>932</b>.

p-0062Antenna control circuit <b>932</b> is another specific embodiment of antenna control circuit <b>132</b> and includes a selection sequence generator <b>976</b> and a switching timer <b>978</b>. Selection sequence generator <b>976</b> produces the antenna selection signal for selecting an antenna of diversity antenna system <b>326</b> to be connected to transceiver <b>330</b> in response to an antenna switching timing signal. The antenna is used for receiving the incoming signal and transmitting the outgoing signal. In one embodiment, selection sequence generator <b>976</b> includes a random sequence generator that dynamically generates a random sequence and produces the antenna selection signal to select an antenna of diversity antenna system <b>326</b> in response to the antenna switching timing signal according to the dynamically generated random sequence. In another embodiment, selection sequence generator <b>976</b> includes a predetermined sequence generator that produces the antenna selection signal to select an antenna of diversity antenna system <b>326</b> in response to the antenna switching timing signal according to a predetermined sequence such as a built-in sequence or a programmed sequence. Switching timer <b>978</b> generates the antenna switching timing signal according to a predetermined schedule specifying times at which a new antenna is to be selected. In one embodiment, the predetermined schedule includes a predetermined period being in a range of approximately 50 milliseconds to 500 milliseconds, with approximately 200 milliseconds being a specific example. Switching timer <b>978</b> generates the antenna switching timing signal on a period basis using this predetermined period. In one embodiment, switching timer <b>978</b> also includes a switching holding circuit that holds the antenna switching timing signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. This prevents possible data transmission errors caused by switching from one antenna to another while a data frame is being received or transmitted. In one embodiment, switching timer <b>978</b> delays the generation of the antenna switching timing signal until an ongoing transmission or reception of a data frame is completed. In another embodiment, switching timer <b>978</b> keeps any change in the antenna selection signal from being applied to switch circuit <b>344</b> until an ongoing transmission or reception of a data frame is completed.

p-0063<figref idrefs="DRAWINGS">FIG. 10</figref> is a block diagram illustrating an embodiment of an external telemetry system <b>1022</b>, which is another specific embodiment of external telemetry system <b>122</b>. External telemetry system <b>1022</b> includes diversity antenna system <b>326</b>, an antenna interface circuit <b>1028</b>, a transceiver <b>1030</b>, and an antenna control circuit <b>1032</b>.

p-0064Antenna interface circuit <b>1028</b> is a specific embodiment of antenna interface circuit <b>128</b> and includes tuning circuits <b>342</b>A-N. Tuning circuits <b>342</b>A-N each provide tuning for a corresponding antenna of antennas <b>342</b>A-N.

p-0065Transceiver <b>1030</b> is a specific embodiment of transceiver <b>130</b> and includes a plurality of receiving modules <b>1080</b>A-N and a switch circuit <b>1082</b>. Receiving modules <b>1080</b>A-N each have an input coupled to a corresponding antenna of antennas <b>342</b>A-N through a corresponding tuning circuit of tuning circuit <b>342</b>A-N. Switch circuit <b>1082</b> connects the output of one receiving module of receiving modules <b>1080</b>A-N to external system controller <b>124</b> according to a receiving path selection signal. That is, the incoming signal processed by one of receiving modules <b>1080</b>A-N is selected for use by external system controller <b>124</b> according to the receiving path selection signal.

p-0066Antenna control circuit <b>1032</b> is a specific embodiment of antenna control circuit <b>132</b> and produces the receiving path selection signal. Instead of selecting one active antenna as in external telemetry systems <b>322</b>-<b>922</b>, two or more of antennas <b>340</b>A-N are active through a telemetry session. Antenna control circuit <b>1032</b> selects an effective antenna by selecting a receiving path including that antenna. Each receiving path includes an antenna of antennas <b>340</b>A-N, a corresponding tuning circuit of tuning circuits <b>342</b>A-N, and a corresponding receiving module of receiving modules <b>1080</b>A-N. For example, one of the receiving paths includes antenna <b>340</b>A, tuning circuit <b>342</b>A, and receiving module <b>1080</b>A. Selecting a receiving path includes connecting the output of one receiving module of receiving modules <b>1080</b>A-N to external system controller <b>124</b>. Antenna control circuit <b>1032</b> includes a signal quality assessment circuit <b>1084</b>, a receiving path selector <b>1086</b>, and a path switching timing circuit <b>1088</b>.

p-0067Signal quality assessment circuit <b>1084</b> produces an indication of quality for the incoming signal processed by each of receiving modules <b>1080</b>A-N. In the illustrated embodiment, signal quality assessment circuit <b>1084</b> includes an incoming frame failure detector <b>1090</b> and a signal strength detector <b>1092</b>. Incoming frame failure detector <b>1090</b> detects incoming frame failures from the incoming signal processed by each of receiving modules <b>1080</b>A-N. In one specific embodiment, incoming frame failure detector <b>1090</b> includes a CRC failure detector that detects a CRC failure from the incoming signal processed by each of receiving modules <b>1080</b>A-N and indicates the incoming frame failure for any receiving module in which the CRC failure is detected. In another specific embodiment, incoming frame failure detector <b>1090</b> includes a lack-of-comma detector adapted to detect a comma indicative of a data frame from the incoming signal processed by each of receiving modules <b>1080</b>A-N during a predetermined time window and indicates the incoming frame failure for any receiving module in which the comma is not detected. Signal strength detector <b>1092</b> measures a strength parameter being a measure of strength of the incoming signal processed by each of receiving modules <b>1080</b>A-N. In one embodiment, signal strength detector <b>1092</b> includes an incoming signal strength failure detector that detects an incoming signal strength failure associated with the incoming signal processed by each of receiving modules <b>1080</b>A-N. In a specific embodiment, the incoming signal strength failure detector is substantially identical to similar to signal strength failure detector <b>650</b>.

p-0068Receiving path selector <b>1086</b> adjusts the receiving path selection signal based on the indications of quality for the incoming signal produced for receiving modules <b>1080</b>A-N. The indications of quality include one or both of the incoming frame failure and the incoming signal strength failure. Receiving path selector <b>1086</b> adjusts the receiving path selection signal to deselect any of the receiving paths in which at least one of the incoming frame failure and the incoming signal strength failure is detected. In one embodiment, receiving path selector <b>1086</b> adjusts the receiving path selection signal based on the measured strength parameters for the receiving paths.

p-0069Path switching timing circuit <b>1088</b> holds the receiving path selection signal while a frame of the outgoing data frames is being transmitted or a frame of the incoming data frames is being received. This prevents possible data transmission error caused by switching from one receiving path to another while a data frame is being received or transmitted. In one embodiment, path switching timing circuit <b>1088</b> delays the adjustment of the receiving path selection signal until an ongoing transmission or reception of a data frame is completed. In another embodiment, path switching timing circuit <b>1088</b> keeps any change in the receiving path selection signal from being applied to switch circuit <b>1082</b> until an ongoing transmission or reception of a data frame is completed.

p-0070<figref idrefs="DRAWINGS">FIG. 11</figref> is a flow chart illustrating a method for operating a telemetry system communicating with an implantable medical device. In one embodiment, the method is performed by external telemetry system <b>322</b>, including of its specific embodiments discussed in this document.

p-0071Signals are transmitted and received using a diversity antenna system including two or more antennas at <b>1100</b>. This includes transmitting an outgoing signal to the implantable medical device and receiving an incoming signal from the implantable medical device. The outgoing signal includes an RF carrier signal modulated with outgoing data frames by the telemetry system communicating with the implantable medical device. The incoming signal includes another RF carrier signal modulated with incoming data frames by the implantable medical device. The incoming data frames are recovered by demodulating the received incoming signal. In one embodiment, ASK is the modulation scheme used to modulate the outgoing signal and the incoming signal.

p-0072An active antenna is selected according to an antenna selection signal at <b>1110</b>. The active antenna is an antenna that is currently used for transmitting and/or receiving signals. The antenna selection signal controls which one or more antennas of the diversity antenna system are active.

p-0073A transmission failure deemed to be associated with a null is detected at <b>1120</b>. The null is known to cause such a transmission failure. Examples of the transmission failure include an incoming frame failure, a response frame failure, and a signal strength failure. One or more types of such transmission failures are detected as an indication of a null by the telemetry system communicating with the implantable medical device. An incoming frame failure is detected, for example, by detecting a CRC failure or a lack of comma from the incoming signal. A response frame failure is detected, for example, by detecting a CRC failure or a lack of response frame from the incoming signal. The response frame is sent from the implantable medical device in response to an outgoing data frame sent to the implantable medical device. The lack of response frame is detected within a predetermined period after the outgoing data frame is sent to the implantable medical device. The signal strength failure is detected, for example, by detecting a sudden drop in a strength parameter, such as amplitude or power, of the outgoing signal and/or the incoming signal. The strength parameter is measured by the telemetry system communicating with the implantable medical device, by the implantable medical device, or both. In one embodiment, a transmitting antenna is selected for transmitting the outgoing signal to the implantable medical device based on the signal strength of the outgoing signal as measured and reported by the implantable medical device, and a receiving antenna is selected for receiving the incoming signal from the implantable medical device based on the signal strength of the incoming signal as measured by the telemetry system communicating with the implantable medical device.

p-0074When the transmission failure is detected, the antenna selection signal is adjusted to select a different active antenna at <b>1130</b>. In other words, when a null is deemed to be encountered, the telemetry system switches from the currently used antenna to a different antenna. It is very unlikely that the null is encountered with two antennas of the telemetry system at the same time.

p-0075The antenna selection signal is held while a frame is being transmitted or received at <b>1140</b>. To prevent data transmission errors, the antenna selection signal causes actual antenna switching when no data frame is being transmitted or received. This is particularly important when the time required to complete an antenna switching is not substantially shorter than the time required for transmitting or receiving a data bit. In one embodiment, an antenna switching is permitted while a data frame is being transmitted or received when the time required to complete the antenna switching is substantially shorter than the time required for transmitting or receiving a data bit. In another embodiment, an antenna switching is permitted while a data frame is being transmitted or received when an error correction algorithm is executed to correct potential data transmission errors.

p-0076The different active antenna is selected according to the adjusted antenna selection signal at <b>1150</b>. This newly selected active antenna is to be used for transmitting and/or receiving data frames until another transmission failure is detected. During a telemetry session, steps <b>1120</b>-<b>1150</b> are repeated to continuously monitor for the transmission failures and switch to a different antenna when a transmission failure is detected.

p-0077<figref idrefs="DRAWINGS">FIG. 12</figref> is a flow chart illustrating another method for operating a telemetry circuit communicating with the implantable medical device. In one embodiment, the method is performed by external telemetry system <b>922</b>.

p-0078Signals are transmitted and received using a diversity antenna system including two or more antennas at <b>1200</b>. This includes transmitting the outgoing signal to the implantable medical device and receiving the incoming signal from the implantable medical device, as discussed above for step <b>1100</b>.

p-0079An active antenna is selected according to an antenna selection signal at <b>1210</b>. The active antenna is an antenna that is currently used for transmitting and/or receiving antennas. The antenna selection signal controls which one or more antennas of the diversity antenna system are active.

p-0080The antenna selection signal is produced for selecting a new active antenna based on a predetermined schedule at <b>1220</b>. In other words, the antenna selection signal is adjusted at times specified by the predetermined schedule. The antenna selection signal is adjusted according to an antenna selection sequence chosen to reduce or minimize the probability for the telemetry system to encounter a null. Depending on how the antenna selection sequence is generated, the new active antenna may be the same antenna that is currently used or a different antenna. In one embodiment, the new active antenna is always different from the antenna that is currently used. In one embodiment, the antenna selection signal is produced based on a dynamically generated random sequence. In an alternative embodiment, the antenna selection signal is produced based on a predetermined sequence. In one embodiment, a new active antenna is selected on a periodic basis. In a further embodiment, the antenna selection signal is held while a frame is being transmitted or received.

p-0081During a telemetry session, steps <b>1210</b>-<b>1220</b> are repeated to adjust the antenna selection signal at predetermined times, such as the predetermined periodic basis. The probability for the telemetry system to encounter a null is reduced because both the location of the nulls and the location of the active antenna vary while the telemetry system communicates with the implantable medical device.

p-0082<figref idrefs="DRAWINGS">FIG. 13</figref> is a flow chart illustrating another method for operating a telemetry circuit communicating with the implantable medical device. In one embodiment, the method is performed by external telemetry system <b>1022</b>.

p-0083Signals are transmitted and received using a diversity antenna system including two or more antennas at <b>1300</b>. This includes transmitting the outgoing signal to the implantable medical device and receiving the incoming signal from the implantable medical device, as discussed above for step <b>1100</b>. The two or more antennas are used to receive the incoming signal simultaneously.

p-0084The incoming signal is processed through a plurality of processing paths at <b>1310</b>. In one embodiment, the plurality of processing paths each include a processing modules each coupled to one antenna of the diversity antenna system to process the incoming signal received by that antenna.

p-0085An indication of quality for the incoming signal processed by each processing path is detected at <b>1320</b>. The detection results in indications of quality each associated with one of the processing paths. Examples of the indication of quality include presence of incoming frame failures and incoming signal strength failures. Such indications of quality are indicative of a possibility that the telemetry system has encountered a null. One or more types of such indications of quality are detected by the telemetry system. An incoming frame failure is detected, for example, by detecting a CRC failure or a lack of comma from the incoming signal. The incoming signal strength failure is detected, for example, by detecting a sudden drop in a strength parameter, such as amplitude or power, of the incoming signal.

p-0086A processing path is selected based on the detected indications of quality at <b>1330</b>. In one embodiment, the selection becomes effective while no data frame is being transmitted or received. During a telemetry session, steps <b>1320</b>-<b>1330</b> are repeated to continuously monitor for the indication of quality and to switch to a different processing path when the indication of quality suggests that the telemetry system has encountered a null.

p-0087In various embodiments, the circuits described in this document are implemented by hardware, software, firmware, or any combination thereof. In various embodiments, the circuits or portions thereof described in this document are each an application-specific circuit constructed to perform one or more particular functions, a general-purpose circuit programmed to perform such function(s), or a combination thereof.

p-0088It is to be understood that the above detailed description is intended to be illustrative, and not restrictive. For example, the implantable medical device can be any implantable medical device capable of communicating with an external system or device via RF telemetry. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.